In recent years, the use of Microfibrillated cellulose (MFC) has been explored in many fields. In the papermaking industry, it has been shown that the addition of MFC can increase the strength of a paper or paper board. When used in paper coating applications, it may replace synthetic or natural binders. MFC can be further used in many other technical fields, such as the food industry, polymer or plastic industry, paint, ceramic ink, rubber industry, cosmetic and pharmaceutical industry.
A dispersion comprising fibrillated cellulose, such as MFC, has the appearance of a highly viscous, shear-thinning transparent gel at low dry contents. The gel has high viscosity making it very difficult to make it flow at low shear rates. This makes it difficult to distribute it in dispersions or suspension as well as other processing through pipes and pumps.
Microfibrillated cellulose (MFC) often form the basis of, or part of composites suitable for applications such as fillers or strength additives in paper or paper boards, in paints, foods and pharmaceuticals. These composites are often formed by mixing or slurrying a pigment or inorganic particle, such as precipitated calcium carbonate (PCC) with MFC.
PCC can be produced by treating a lime suspension (calcium hydroxide) with carbon dioxide (CO2). The chemical reaction in which calcium hydroxide reacts with carbon dioxide and forms calcium carbonate is oftentimes referred to as carbonation, or carbonatation.
PCC/MFC composite materials are useful in various industrial processes, such as papermaking, to e.g. increase the filler content without deteriorate the properties and to improve the retention of each individual component. U.S. Pat. No. 6,156,118 discloses a process wherein nanofibers are added to PCC forming a PCC/MFC composite material.
More recently, precipitation of PCC on nanofibers by mixing MFC with lime milk followed addition of carbon dioxide has been described. This, and the use thereof in papermaking, has been disclosed in e.g. U.S. Pat. Nos. 6,251,222 and 5,824,364. WO2014072913 discloses a process for production of a dry composite material, which is obtained by mixing MFC with lime milk and subsequently adding carbon dioxide to induce calcium carbonate to precipitate on the MFC fibrils. EP2486188 further describes a process wherein milk of lime and nanocellulose were pre-mixed whereupon carbon dioxide was injected.
However, when adding MFC to milk of lime prior to the carbonation, PCC tend to substantially completely cover the MFC fibrils, whereby the strength enhancing properties of the MFC is deteriorated. Moreover, the opacity of the thus formed composite has not been satisfying. It is further difficult to achieve an even distribution of the added calcium dioxide during carbonation, which results in an uneven reaction and quality defects in the end product. Another problem related to this process is that the MFC tend to form flocs due to its high viscosity.
Therefore, there remains a need for method to form a composite comprising MFC and PCC, which method eliminates the aforementioned problems.